When drilling a borehole through a geologic formation, it is important to know the downhole conditions to ensure that the drill bit is operating correctly. These conditions include, among other things, the diameter of the borehole and, therefore, the volume of the drilling fluid at any given point. In addition, the formation properties may be measured to predict the presence of oil or gas. Formation properties may be logged with wireline tools, logging while drilling (“LWD”) tools, or measurement while drilling (“MWD”) tools. Modern oil and gas explorations typically use LWD or MWD tools, instead of wireline tools, for formation logging due to the saving in time and costs.
Various LWD and MWD tools are in use for measuring borehole or formation properties. For example, LWD neutron or gamma spectroscopy logs may be used to provide lithology, formation porosity, and formation density information. Neutron/gamma spectroscopy may be performed by sending a pulse of neutrons into the formation using a pulsed neutron generator (“PNG”). The neutrons interact with elements in the formation by inelastic interactions or elastic interactions. The high-energy neutrons gradually lose their energy through these interactions to become thermal neutrons, which may be captured by the nuclei of various elements in the formation. After neutron capture, these elements become activated. The activated elements then decay by emitting gamma rays. The gamma rays emitted by these activated elements may be detected with gamma ray detectors. Because different elements produce gamma rays of different energies, the captured gamma ray spectra may be used to derive the elemental compositions of the formation. The elemental yields in turn may be used to provide formation lithology because different sediment layers are typically enriched with different types of elements.
Methods for neutron and gamma ray logging may be found in, for example, U.S. Pat. No. 5,440,118 to Roscoe, U.S. Pat. No. 5,786,595 to Herron et al., U.S. Pat. No. 5,539,225 to Loomis et al. and in Albertin et al., “The many facets of pulsed neutron cased-hole logging,” Schlumberger Oilfield Review, Vol. 8, No. 2, pp. 28-41 (1996).
In borehole drilling, large quantities of drilling fluids are pumped through the drill strings into the boreholes while the boreholes are being drilled. The drilling fluids help cool the cutting surfaces of the drill bits and help carry out the earth cuttings from the bottom of the borehole when they flow up the annulus to the surface. To prevent formation fluids from flowing into the borehole during the drilling process, the drilling fluids are pumped under a pressure that is slightly higher than the expected formation pressure. The higher hydraulic pressure of the drilling fluids may result in a substantial loss of fluid into the formation when a permeable and low pressure zone of the earth formation is encountered. Detection of such fluid loss may be used in correction of the measurements of various LWD sensors. Fluid loss into the formation may be detected by the reduced flow back of the drilling fluids on the surface. However, for determining in what zone the fluid loss is occurring, means of detecting volumetric flows along the axial depth of the borehole may be needed.
During periods, such as drill pipe connections, when drilling fluid flow is stopped and then restarted, the bottom hole pressure is less than the flowing pressure, due to the absence of pressure drops in the annulus due to flowing friction. It is often indirectly observed from downhole pressure measurements that when drilling fluid flow ceases, additional fluid flows out of the formation—which is then taken up again on the resumption of fluid flow from the surface. This phenomenon is known as wellbore breathing, and an assessment of the pressures and volumes involved can be used in inferring properties of the formation. However, inference of the fluid volumes and rates observed is difficult due to the lack of a direct flow rate measurement in the annulus. Measurement of the annular fluid flow rate over short periods, such as those occurring during changes in the surface flow rate, require a continuous measurement of annular flow velocity.